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Airfoil Analysis (Continued)
Wow, this was a lot of work. A lot more than I expected going in. However, it was also one of those engineering exercises where hard data provides big surprises and overturns initial expectations. I’m glad I put in the time and effort.

And this is a very long post. Sorry about that, but it all needs to be here.

Selecting my five candidates above was a process of taking a larger group of suggestions and running comparisons to see which seemed to meet my overall requirements. Airfoiltools.com provides a comparison tool that runs airfoils through a basic XFoil analysis and displays several different aspects of performance for the airfoils being compared.

Here’s a sample run for the Wortmann FX 67 (tan/light curves) and Wortmann FX 79 (green/dark curves) on AirfoilTools. This comparison was made at a Reynold's Number (Rn) of 1,000,000 and an Ncrit value of 9.

Clearly, while both meet my raw requirements, the newer FX 79 is a much better choice, with a zero-alpha pitching moment (Cmzero) magnitude of nearly half that of the FX 67, and a higher maximum lift coefficient (Cl). The one potential issue is that the drag bucket of the FX 79 (shown on the Cl/Cd curve) ends very close to, or maybe even less than, the target of 0.93 in my requirements.

I ran a fairly large number of comparisons to arrive at my final five candidates, sometimes three and four airfoils at a time, comparing curves and eliminating the worst ones until I had these five.

AirfoilTools is primarily intended for modelers, and the range of available Reynold’s Numbers tops out at 1,000,000. To select a final winner, I ran a number of datapoints directly through XFoil myself. I know a lot of you like XFLR5, or Profili, but both of those are actually front-ends for XFoil. I don’t mind XFoil’s command-line interface, and I happen to prefer XFoil’s method of exporting polars, as a postscript file. My tools as a graphic artist allow me to work with postscript files very easily.

One thing I want to emphasize at this point is that these runs are not to collect definitive data on these airfoils. Instead, I’m comparing them under certain conditions that will be present for my airplane at notable parts of the flight envelope. So, for example, you’ll find I’m listing Cdmin values, but then getting airfoil L/D at a different lift coefficient. At this point, I just want to see which of these airfoils is the most suitable, and then I’ll dig out exact performance numbers and comprehensive polars for the winner later.

Just to make sure I was proceeding with an apples-to-apples comparison, all the runs were done as follows:

All data files were de-rotated (DERO command) and normalized (NORM).

All data files were converted to 140 panels using PPAR.

Analysis was done in viscid mode, with Reynold’s Number and Mach number set to the values in the table below.

Ncrit was set to 11 for all of the simulations except those with roughness, where it was set to 6.

Modern computers are powerful enough that XFoil runs very quickly, so I set the ITER limit to 999, its maximum value.

For the “L/D” fields, I calculated the average airfoil lift coefficient at my estimated L/Dmax and cruise speeds, for weights appropriate to each case. Without replicating those calculations here, the airfoil Cl at these weights and speeds came out at 0.60 and 0.22 respectively. I ran a simulation of each airfoil at each Cl, and noted the airfoil L/D, which is a value XFoil presents as part of each single-point run.

For Cl in the flaps-down case, I arbitrarily selected a flap deflection of 15°. The goal here was to see if each foil had the capability to reach at least close to the desired value at a “reasonable” deflection, without having to run a bunch of tests to find which deflection generated the desired Cl - something to be done later. For all ‘foils, the vertical location of the flap hinge was mid-way between the upper and lower surface. The chordwise location of the hinge was 0.8c (80% of the wing chord), corresponding to the 20% chord flaps in the current layout. I did two runs with flaps down: One at Rn=500,000 for the wingtip, and one at Rn=900,000 for the taper-break at the middle of the semi-span.

The raw data from these XFoil runs is tabulated in “Airfoil Performance Comparison” below.

While rain is unlikely to be an issue for this day-VFR-only airplane, it always seems like I manage to take off through a cloud of gnats on a really great soaring day, and the splatter of remains over the leading edge degrades the performance of the airfoil somewhat. The first laminar-flow airfoils suffered big increases in drag and big losses in lift when “dirty”, the latter resulting in a trim change as the wing had to be at a higher angle of attack to produce a given amount of lift. Modern laminar airfoils are much better, but there is still a degradation of performance when the wing is dirty. I wanted to compare “clean” and “dirty” performance for the airfoils to see which lost the least amount of performance after an encounter with the Gnat Patrol. To simulate a thoroughly-contaminated condition, I set fixed laminar-to-turbulent trips in XFoil at 0.05c on the upper surface, and at 0.1c on the lower surface. To better simulate the turbulent conditions, I reduced Ncrit from 11 to 6. Thanks to all who contributed to the chat in the discussion thread that led to this method. I then did a run on each airfoil at Rn=1,600,000 and Mach=0.07, which is the condition at the taper break on the wing when the airplane is soaring at L/Dmax. I have values for Cdmin, angle of attack, and airfoil L/D for this condition “clean”, so now I re-ran those values with the trips and reduced Ncrit. The results are in the table “Airfoil Performance Comparison, Clean and with Roughness.”

Lastly, I ran a polar of each airfoil at a Reynold’s number of 1,200,000 and a Mach number of 0.05, which are flaps-up stall values at the taper-break near the middle of the semi-span. This is to show airfoil stall behavior. The polars are shown in “Polars for Stall Behavior” below. I’ve circled the area I’m looking at in blue on the graph for the first one, the FX 79. Note how the Cl versus ɑ (angle of attack) curve gently rounds over the top before diminishing in value? That’s a nice, soft, airfoil stall. One that had a sharp break would be a hard airfoil stall. For this point in the process, I just care about the shape of the curve at the stall.

Results

Airfoil Performance Comparison

Airfoil Performance Comparison, Clean and with Roughness

Polars for Stall BehaviorNote: This is flaps-up stall behavior.

As you can see, all my candidates have a soft stall. Since airplane stall characteristics are much more a function of wing geometry than airfoil stall characteristics, I will have no troubles with any of these. Additionally, since they all are soft-stall airfoils, I won’t have to make much in the way of performance-sapping tweaks to the wing geometry in order to tame a hard-stall airfoil.

Discussion and Choosing a Winner
As I alluded to earlier, the end results of this work surprised me a bit. I’ll also confess to a bit of a fib - the order of the candidates I posted when I listed the candidates wasn’t random. They were posted in the order I thought most-likely to be how they ranked in the final results. I expected the modern Wortmann FX79 sailplane airfoil to be the clear winner, on down to the Riblett GA40415 which I expected to simply not compare to the others. Well, hard data told a different story.

While there weren’t any clear winners among the candidates, there were two clear losers: The Roncz 1082T and the Eppler 662.

This one was a big surprise. While all of these airfoils protested a bit at being operated at Reynold’s Numbers below about a million, the Roncz 1082T hated it. Passionately. The airfoil only gained 0.25 in Cl from a 15° flap deflection at Rn=500,000, and the angle for Clmax dropped all the way down to 11°, a clear sign of early separation due to low Rn. The drag bucket fell apart below a Rn of one million, and the airfoil’s Cd rose above 0.01 at any Cl greater than 0.5. Above a Rn of one million, the airfoil had very good performance in all categories, so it’s not that this is a lousy airfoil - it performed very well on Voyager, after all - it’s that it was designed for a particular operating condition, and the low-Rn operating environment required by my little motorglider isn’t that condition. So strike the 1082T from the list.

The other early casualty was the Eppler 662. The airfoil performed exceptionally well in terms of drag, even with roughness “turned on”, equalling even the modern Wortmann FX79 in terms of minimum drag, and developing a much better airfoil L/D at my airplane’s cruise condition. That aft camber really pulls the laminar flow aft on the upper surface, creating a long laminar run under nearly all conditions. But oh, the price you pay. Look at the Cmzero numbers. Pitching moment coefficient is easily twice that of even the worst of the others. This is simply a function of the airfoil’s age - airfoil design methodology when this airfoil was designed emphasized minimum drag at any cost, and trim drag wasn’t really considered. Given that my airplane has a relatively short tail compared to full-blown sailplanes, I simply can’t afford all the trim drag this airfoil would cause on my airplane. Strike this one.

Next down was another surprise, the Eppler 642. My earliest testing in AirfoilTools.com indicated that this one might actually be the winner, but again Reynold’s Number played havoc with that idea. Below a Rn of about a million, the drag bucket pretty much disappeared entirely, with the airfoil showing a Cd of ~0.01 except for one little dip to about 0.008 at a lift coefficient near zero. At higher Rn, this was a fairly good airfoil choice, but the low-Rn behavior means that in weak thermals, my little glider would be seeing a drag rise that would penalize performance. So, the expected winner was actually an early cut.

Cutting those three left the Wortmann FX79 and … the Riblett GA40415! I don’t know why, but I expected the Riblett ‘foil to simply not compare to these high-performance sailplane airfoils and the ultra-specialized world-cruising airfoil from John Roncz but, for my test conditions, the hard data clearly said otherwise.

I’m having a very hard time deciding between these final two. Both are excellent contenders, but both have minor issues. Let’s go through those, and hopefully writing it down will help my thinking. It's another reason why I'm posting this online.

The FX79-K-144/17 generates a little more lift with flaps down at low Rn, and it gets to Clmax at a lower angle of attack than the GA40415 in most cases. The latter is potentially important because it allows a smaller amount of upsweep in the aft fuselage, aligning it better with the downwash field coming off the wing. That means less drag from the fuselage. However, the drag bucket of the FX79 is a little narrower than the GA40415, to the extent that some part of the wing is likely pushing out of the bucket at two conditions: minimum-sink soaring and at the end of cruise on the design powered reference mission - especially if the airplane is carrying a light pilot and/or no baggage. The sailplane-like solution to this is to make my flaps/ailerons a true camber-flap arrangement, going down slightly at minimum sink, and reflexing slightly at cruise. Both operations move the drag bucket to where it’s needed, but it’s an operational complication and putting flaps down at minimum-sink means a trim drag penalty at that condition, since flaps-down increases pitching moment. On the other hand, if the airplane is light during minimum-sink soaring, the Cl will be lower and the flaps-down under that condition may not be necessary. I could specify that payload (especially baggage and fuel) be limited in soaring missions, which is not unreasonable. An operational restriction to cover an aerodynamic one.

The GA40415 has a wide-enough drag bucket to allow undeflected flaps under nearly all conditions, which is simpler for the pilot and the designer - scheduling flaps and ailerons through all those motions requires a complicated mixer, although I’ll need some kind of mixer for flaps-down regardless, since the ailerons are going to droop, too. The downside of the GA40415 is the high angle of attack at which it develops Clmax, as much as 16° in the case of the mid-span, flaps-down. For reference, the FX79 stalls at 13° in the same conditions. While sailplanes routinely land well above stall speed, motorgliders operate more like power planes in this regard, and a stall angle this high means either a taller heavier main gear, more drag-producing upsweep of the aft fuselage, or both. This airfoil also has some low-Rn early-separation problems in the flaps-down stall conditions at my wingtips. I also note that the GA40415 is close to falling out of the low-Cl end of the drag bucket at the end of the design powered cruise mission, meaning it’s possibly going to need reflex as well.

Writing all this down makes it clear to me that the GA40415 has more problems for my specific use-case. Both airfoils may require some reflex at cruise, so that’s a wash, and their performance both clean and dirty is pretty much identical otherwise. I’m choosing the Wortmann FX 79-K-144/17.

Airfoil chosen.

Next Post: Redrawing the aircraft for final analysis and optimization.

Super Moderator

Redrawing the Design
It’s time to get the three-view drawing of my design all caught up with the numbers I’ve been crunching. If you’ll recall, the drawing as it stands now is little more than a very fancy “back of the envelope sketch”. The wing is pretty darned close to the numbers, but the fuselage is very much “notional” - an airplane-shaped sketch with the places for the major items staked out.

​

The next stage in the design process requires that I pin down a rather large number of measurements, so I need to make sure those measurements work on the plane and everything fits in reality. While I’m at it, it’s time to answer some questions that have come up and incorporate the answers to those questions into the design. To that end, I’ve been working up a list of things I want to check, and things I want to decide and act upon when redrawing the airplane.

Checking Assumptions

Check all dimensions for wing. Make sure the drawing really is accurate.

Check assumptions on fuselage length. How much space do I really have?

Check assumptions about overall height, including vertical fin. How much space do I really have?

Check the scaling of the engine drawings. Review some photos and see if I can make the drawing more complete/accurate.

Check the size of my "human figure" and replace with combined figures for the limits of design pilot size.

New Work

Replace wing section in side-view with Wortmann FX 79-K-144/17.

Choose tail airfoils, or at least families. (No, this isn't as drawn-out as for the wing!)

Decide upon crashworthiness strategy and develop tactics to an appropriate level to show at this time.

Am I going to include space for a parachute? Pack (on pilot), or BRS?

Make sure the fuel tank can accept my entire fuel load, as-drawn.

Can I reduce canopy size a little, to keep costs down, without compromising view or aesthetics?.

Redraw firewall-forward. Use real spinner shape from Aircraft Spruce, try to extrapolate engine shape better from Scott Casler's drawings and those from Great Plains.

Decide if the flap continues all the way to the fuselage side.

Where does the wing dihedral break happen? At the centerline, or where the outer panels remove?

Ensure that the horizontal tail is easily removable for trailering.

Try to move the horizontal tail aft as much as possible, so that it's smaller.

Try to get the bottom of the fuselage completely flat, for ease of manufacture.

I’m sure I’ll come up with some more as I go along, but that’s a good start.

Super Moderator

Redrawing - The Cockpit
While I'm gathering confirmed dimensional data, I can start in on the rest of the redraw work. Let's start with the cockpit, and let's start with compound fractures. Crushed vertebrae. Death.

It's time to talk a little bit about crashworthiness.

I like my body. I'm not conceited about it, and I've no reason to think it's any better than anyone else's body, but I've become rather attached to it over the 51 years we've been hanging out together. I'd like to hang on to it a while longer. There's a school of thought that says the best way to survive a crash is not to have one, and I'm a firm believer in that philosophy. But airplanes somehow manage to crash anyway, and it would be really dumb not to make some provisions to improve my chances of survival, should my little airplane accomplish it.

I've spent the last few weeks studying small airplane crashworthiness more extensively than I have before. There are regulations regarding crashworthniness in FAR 23 and EASA CS 22 (the European glider and sailplane certification regulations). There are a lot of research materials available out there detailing aircraft crashes, how and how many people were injured or died. There are a lot of materials out there detailing airplane loads and structural analysis. Unfortunately, there seems to be very little out on the open Internet detailing design methodology for crashworthiness. The best resource I was able to find were some discussions right here on HBA, and a book-size report from NASA's AGATE program, Small Airplane Crashworthiness Design Guide. I've used the latter extensively as I researched the subject for my project.

At this point in the design process (conceptual design) it's appropriate to decide and describe the elements of the overall system I'll employ in the DS54, and the general requirements the system has to meet. Developing the engineering details of each element belongs in later phases of design, so I won't be doing that here. Throughout, however, I have to keep weight in mind. "Weight is the Enemy", as Billski so often and accurately reminds us, and while I do have some weight margin in my design, I need to keep weight in mind at all times, lest I use up that margin and start eating into performance.

TYPES OF IMPACTS
First let's take a look at the enemy. Airplanes crash in certain ways that impart certain predictable loads to the pilot, that have to be protected against.

Longitudinal Impacts
There are two types of longitudinal impacts (click on the image for a larger version):

The first is how low-altitude stall-spins end. Nose into the ground at a steep angle. This is what killed my friend Bill.

The second type happens after an emergency landing, with the aircraft sliding into an obstacle. This can generally be made survivable. If you take the ground away below, it's also flying into the side of a mountain at full cruise, but I have seen nothing to say that, with current technology, a full-cruise longitudinal impact could be made reliably survivable. Fly directly into the side of a mountain and you're pretty much done.

Vertical Impacts
The human body can withstand a much higher load perpendicular to the spine than parallel to it. High loads parallel to the spine generally result in crushed vertebrae, which is a really nasty injury, with a long hospital stay certain and lower-body paralysis a distinct possibility. It might be tempting to think that vertical impacts don't happen very often in fixed-wing aircraft, but they do. Again, there are two common types:

The first involves a normal approach to emergency landing. The pilot over-flares, and the tail strikes the ground first. This "slaps" the nose of the airplane down, hard. In fact, this is one of the most dangerous impacts there is, highly likely to result in spinal damage to the pilot in most airplanes. The second is typical of continuing a spin - especially a flat spin - to the ground. The nose strikes first and slaps the tail down. This can also happen at the end of a low-altitude stall-spin, if the pilot manages to get the nose up a bit so that it slides forward on the ground instead of digging in. Given the likelihood of severe spinal injury, it's debatable if that's an improvement.

Top Impacts
Inverted spins to the ground generally can't reliably be made survivable from what I can see, so what we're talking about here is a flip-over in the course of an emergency landing. The nose digs in or snags on something on the ground, and flips the aircraft over onto its back. Because of the wings, it's less common for an airplane to flip over sideways, but it can happen.

The big trick here is protecting the pilot's head. The impact speeds are moderate, but our heads are very fragile things. This kind of crash is a great argument for wearing a helmet, but that's a personal choice.

It's also possible for an airplane to slide sideways on the ground into an obstacle, generating a significant side load. I didn't make a drawing for that one.

Super Moderator

LOADS MODEL
A loads model describes the loads the crashworthiness system is designed to mitigate. Picking one allows me to design to that model, just like setting overall requirements allows me to design an airplane to those requirements. So, I went and looked up what the loads model should be. And ran into a brick wall.

NASA and USAF researchers (among others) have determined approximate limits of human tolerance to "g" loadings. (Even those are considered too low by some in the field). FAR 23.561 sets overall design loadings that the airplane structure should be able to withstand, then FAR 23.562 comes along next and sets another, higher, set of loadings for testing airplane seats. EASA CS 22 has its own - somewhat different - set of loadings for the aircraft structure, and says nothing about loading requirements for seats in an emergency landing situation. The NASA/AGATE report I referenced in my last post has very specific recommendations for both static and dynamic loadings, and those recommendations are completely different than all the others. It seems there's no actual consensus in the industry regarding what "emergency landing" loads we should use to design our airplanes, which is really incredibly frustrating.

Here are the major static loads standards or recommendations I have been able to locate for light aircraft design. The aircraft should be designed to handle these loads without significantly distorting the cockpit cell, ripping out pilot restraint mounts, rupturing the fuel tank, etc. Attenuation of loads is done separately.

Table - Various Design Static Crash Loading Specifications

LOADING

CS 22.561(b)(1)

FAR 23.561(b)(3)

NASA/AGATE

Forward

9 g

9 g

26 g

Downward

4.5 g

N/A*

16.5 g

Upward

4.5 g

3 g

4.5 g

Sideways

3 g

1.5 g

4.5 g

*FAR 23.561(b)(2)(iv) requires a 6 g downward loading, but only when certification to the emergency exit provisions of §23.807(d)(4) are requested.

In the end, I've decided to adopt the NASA/AGATE recommendations. Having read through FAR 23 and EASA CS 22, I don't feel that they're even remotely adequate to providing even minimal pilot crashworthiness protection. Who cares if the seat can protect the pilot in a 19 g longitudinal impact if the fuselage is allowed to fold up and kill him at 9 g's? The opposing pole is that, if the design crash loadings are set to full-blown human tolerance, mitigating them will add so much weight to the airplane that it won't meet design specifications for the project, or maybe even fly at all. The loading a well-supported human body can withstand in the chest-to-back direction without serious injury, for example, is at least 45 g, and I've been told the real "limit" is probably as high as 100 g. The one modification I'm going to make to the NASA/AGATE recommendations is to bump the upward and sideways design loads up to 6 g, from 4.5 g. This is based on the recommendation of someone familiar with the issues involved in aircraft crashworthiness. In neither case is a significant weight penalty involved, so why not?

Super Moderator

Between the discussion thread and my work here, I was thinking I was pretty much ready to start writing out mitigation strategies and tactics I'd use on the DS54 to absorb emergency landing loads. I have them listed out in my design notebook, I had them all visualized in my head, and "just" needed to confirm the visualization on my developing to-scale redrawing of the aircraft.

It doesn't fit. Like, "at all" doesn't fit. The geometry between the pilot's rear end, the seat pan, and the main spar carry-through means the latter is sitting exactly where my seat will stroke in an emergency landing. I already have concerns (to be proved or disproved shortly) that the wing is slightly too far forward for realistic CG locations, so moving it forward to get it out of the way isn't an option. I tried moving the wing down and/or the pilot up. On this small of an aircraft, the result has all the fine aesthetics of a WWI French main battle tank, and is about as aerodynamic. This is where scale drawings are critical. In my mind's eye, this all worked. In reality, it doesn't. Big difference and, when in doubt, get it down to scale in some kind of drawing!

I really don't know what to do. At the moment, I have a fixed seat pan drawn in, and thickened up the seat-pan energy-absorbing foam pad a bit. While I don't like it at all, I may end up having to go with the sort of sailplane practice Autoreply mentioned in the discussion thread, and use the extended main landing gear as the primary load-attenuator for vertical impacts. I'd far rather have an under-seat attenuator for the reasons I listed out during that discussion, but not at the expense of an airplane that won't meet the mission specifications. Because, if it won't meet the specs I need, why build it at all?

I'm posting this from a restaurant on my tablet and can't render out the Adobe Illustrator drawing to something that can be posted here, but I'll try and do so tomorrow and edit it into this post then. The rest of my day is already promised to other tasks.

Super Moderator

After a lot of thought and an extended discussion over in the discussion thread (approximately posts #520-565), I've made up my mind and am moving forward. While I would very much like to have an under-seat impact attenuator, there simply isn't room in this design without otherwise compromising performance significantly - possibly even pushing some of the soaring parameters below their threshold values in my requirements set.

Like every other airplane in this class of which I'm aware, I'm going to use the landing gear as the primary impact load attenuator, and a TempurFoam (now called "Confor Foam") seat pad to soak up the remaining loads. While it's not the level of protection I would've liked it should be adequate, provided the gear is down during a vertical impact. I can increase the odds of that with POH operational guidelines. For example, I can mandate that the gear be extended no later than the downwind leg of the pattern (approach stall-spin accidents), on climb-out to, say, 500' AGL (takeoff stall-spin), and during "new" ridge-soaring encounters, either by pilot experience or by location (low-altitude stall-spin accidents while ridge soaring).

Here's the famous inboard profile for the Quickie 1. In very many ways, especially simplicity and construction technique, you can think of my airplane as "a long-winged Quickie, intended for soaring as well as powered-flight." That's pretty much how I think of it. Where my spar carrythrough is, the Q1 has the main fuel tank. The landing gear provide a little impact load attenuation, but they're really long levers, too, so I'm not sure how well that really works.

My airplane will be no worse than this and, in fact, will be considerably better since I don't have fuel in the bottom of the fuselage, have a purpose-built crash-cage cockpit cell, plus a forward crush zone that the Q1 doesn't have.

Super Moderator

Wow. My last post was in April. Now it's August! Lots of things going on, and you may have noticed that my participation here (at all) has been sporadic of late. Fortunately, some of the "busy-ness" was lots of business, but some of it wasn't. I've gotten both over some hurdles, and I've got a little more time to devote to airplanes, and this thread.

What follows continues the general technology-choices discussion I need to finish before moving on into more-detailed design calculations on the airplate. Most of this very long post was already written back in April, but rather than hold off until it's all done, I'll split it into two parts.

I'll go down the list one by one with how I've decided to resolve each item.

Container
The cockpit structure surrounding the pilot provides the first line of defense against impacts. Nothing else matters if the cockpit structure folds up and kills the pilot itself. The goal is to make the cockpit structure into a "crash cage" that will resist significant deformation for loadings up to the design impact loads, in each direction, and under likely loading points. For example, if the fuselage is "ploughing" through soft earth, there will be a very high shear load applied to the cockpit, top-to-bottom. A flip-over impact means some kind of "roll bar" is necessary to protect the pilot's head and upper body.

There has been a lot of debate here about the suitability of composites for crash-cage structure, most of it surrounding the tendency of composites to "let go" at their structural limit with very little energy absorption; "shattering", as it were. I'm going to use a composite cockpit structure, but bypass the issue by not using any part of the cockpit structure for energy management at all. It'll be designed to take impact loadings up to the design impact loads. Energy management will be handled by other structures. An extension of the main bulkhead after of the pilot provides roll-over protection.

Restraint
I've done some amateur auto racing, and I know the value of a five-point restraint harness. The point of the harness is to keep the pilot in place and from being beaten to death against the inside of the cockpit structure. Aircraft harnesses should be TSO-C114 and AS8043 compliant, and have the following strengths: Lap belt - 5000 lb, shoulder belts - 4000 lb, belt tie-down strap - 2000 lb. Tie-down anchors should all be able to withstand 1.5 times maximum webbing load (see above), within a 30° angle in any direction of the normal seated geometry of the belt.

The seat itself is a reclining, semi-supine seat partially built into the aircraft structure. The seat back is the main fuselage bulkhead. The seat pan bridges the bottom of the seat back and the main spar. (EDIT: The following greyed-out sentence is no longer true, and I should've edited it out prior to making this post. Thanks Himat, for spotting this!) The seat pan is a separate part, mounted on an impact attenuator, and capable of stroking down 3". The seat pan is angled to provide significant support to the pilot's legs, and to help prevent him from "submarining" forward under the lap belt restraint.

Energy Management
With the pilot protected by the cockpit cage structure, and held safely in position within it by the seat and restraints, it's time to look at dissipating the crash energy.

The first tactic is to minimize the loads from the start. One of the big sources of longitudinal loads is the lower edge of the firewall digging, or "ploughing", in to the ground as the airplane slides along on its belly. The pile of dirt that builds up in front of the airplane slows it extremely rapidly, building up a very high "g" loading. The solution is to angle the firewall to help the airplane "ski" on top of the soil. I've included an angled firewall in my design, and may further chamfer or curve the lower edge if it's possible.

After that, loads and crash energy are dissipated by "crush zones" in the structure, much as automobiles do. The energy of the crash is expended in deforming structure over a working distance.

For longitudinal loads, there is an obvious crush zone at the front of the airplane - the engine mount. By allowing the engine to be shoved back towards the firewall, the strong tubing that forms the mount can be used as a crush zone to absorb energy in a crash landing. Unfortunately, there's not a lot of room there before the starter and magneto on the back of the engine slam into the firewall, so I've come up with what I hope is a clever way to increase that distance: Let the engine rotate prop-up as it moves aft. Here's the "normal" position of the engine:

While this is little more than a schematic representation at this stage, note how the tubes of the engine mount are not the same diameter. I'm taking advantage of the tendency of narrow tubes to buckle in compression before wider ones to "stage" the failure of the mount in a specific order. The top tube is the narrowest, and should buckle first, absorbing energy as the engine rotates prop-hub upwards. The diagonal tube helps brace the bottom tube while this is happening, before it starts to buckle and fail, absorbing still more energy as the whole assembly pivots upwards on the thick bottom tube. The end result gives the front crush-zone considerably more effective depth (over a foot in total), and the end result should look something like this (I've left a copy of the spinner and the prop to show their original positions):

Obviously there's more to it than just the diameter of the mount tubes. They're of unequal lengths. so I'll need to take that into account as well, since a shorter tube buckles at a higher load than a longer one, all else being equal. Figuring out the necessary diameters and wall thicknesses to not only function as a strong mount, but also stage failure in the way I want, will be a task for a later phase of design. This is also a fairly idealized system. In the real-world, loads will be asymmetric, and not perfectly aligned with the centerline of the aircraft. Margins of safety in the "staging" will have to take that into account, as best as possible, but some asymmetry in the collapse of the mount is to be expected.

Attenuating vertical impacts proved more of a challenge than I initially expected, as we talked about over in the discussion thread. I ended up with a two-fold strategy for these loads: First, the main landing gear "legs" form a crush-zone, as they collapse in overload back up into the wheel well. One point I'll need to address is staging the failure of the tubes so that they don't expand outwards as they fail. The pilot's legs are on either side of the main gear well, so the tubes that make up the landing gear struts will need to fail inwards and upwards as they collapse. The second part of the vertical impact strategy is energy-absorbing foam for the pilot's seat. This is the modern version of the "astronaut foam" that NASA developed for the Apollo program, and it's very good at attenuating impact loads.

Next Post: Continuing down the list: Environment and Post-Crash factors.

Super Moderator

I'm really a bit distressed at how big this diversion into crashworthiness has become. Obviously critical work, but my project has lost focus and needs to get back on-track. The big problem here is, I think, that I've been too wordy about describing what I'm doing. For this last post, I'm just going to summarize the remaining "Environment/Post-Crash Factors" choices in bullet-point lists, and please ask for any clarification on the material in the discussion thread. What you see below is much closer to how the material actually appears in my design notebook.

So, without further ado...

Anti-ploughing Structures
The nose of the aircraft digging in and "ploughing" the ground is a major source of deceleration force. A curved or chamfered lower firewall segment acts as a "ski tip" to reduce ploughing. If possible, a 'rocker' curvature continues from there to aft of the cockpit. Fuselage bottom surface smooth, and sufficiently thick and tough to resist punctures from objects on the ground as the aircraft slides. Check lower engine mounts and/or crush-zone structure for possibility of "digging in". Overlapping exterior skin panels should "shingle" forward to aft, to prevent a leading edge from digging in.

Seat

Confor Foam pad in the seat pan for energy absorption.

The seat back is fixed to the rear cabin bulkhead, and is covered with a full-length Confor Foam pad.

Loads parallel to the pilot's spine are absorbed by collapsing the main gear legs, and by a Confor Foam seat pad.

Seat strength overall is sufficient to hold together in a 26 g impact.

In a sideways impact, the seat should not deflect more than 2" in a 6 g impact. Side bolsters can absorb some of the load into the cockpit sidewalls.

A significant leg bolster helps prevent submarining under the lap-belt, possibly aided by a five-point harness.

Anchor Strength: 1.5 times maximum webbing load (see above), within a 30° angle in any direction of the normal seated geometry of the belt.

Interior Crashworthiness

Keep the instrument panel 30" or more forward of the rear of the pilot's head in the normal seated position. (Flail criteria). If this isn't possible (pilot reach to instrument controls), look at an energy-absorbing glare-shield solution to mitigate impact to the pilot's head.

Side-stick primary control. Less danger of impaling the pilot's head or chest on the stick. Also easier to run the aileron controls as a torque-tube under the pilot's arm.

Design rudder pedals to not trap the pilot's feet. A "simple bar" is undesirable. Pedals should contact both the ball of the foot and the heel.

Smooth interior cockpit panels, all the way down into the footwell.

Objects in the cabin should be rounded or padded where practical.

Fuel Location and Containment

The current location - mounted on top of the forward crash cell, over the pilot's legs - provides protection for the tank against distortion and subsequent leakage in most impact situations (inverted may possibly present a problem). A sheet-metal tank is my preference.

Structural stiffness in the upper-forward fuselage under the tank will be a concern, to reduce the chance of distortions to the structure rupturing the tank.

I want to provide explicit separation between the fuel tank and the inside of the cockpit. If the tank leaks, I want no fuel to enter the cockpit proper. This can be as simple as a thin steel sheet (aluminum has too low a melting temperature if the fuel leaks and ignites), or I can have a closed upper structure here and cover the upper surface with Fiberfrax as a "secondary firewall".

Check all fuel line routings for possible normal-use chafing, or cutting, pulling trapping in a survivable crash. Steel-mesh lines are preferable. See remainder of considerations on AGATE report's General Aviation Crashworthiness Design Evaluation section, pages 24-25, see also Appendix C, same report.

Super Moderator

Yeah, moving really, really slowly here. Just lots of work for my business, lots of "life" going on. But I've been working on the project here and there, and I can at least let you know where I'm at on it. Here's the list from post #62 above, with annotations for status.

Checking Assumptions

COMPLETED: Check all dimensions for wing. Make sure the drawing really is accurate.

COMPLETED: Check assumptions on fuselage length. How much space do I really have?

COMPLETED: Check assumptions about overall height, including vertical fin. How much space do I really have?

COMPLETED: Check the scaling of the engine drawings. Review some photos and see if I can make the drawing more complete/accurate.

COMPLETED: Check the size of my "human figure" and replace with combined figures for the limits of design pilot size.

Super Moderator

It's been nearly a year since I made a post on this project. In the interim, to say that "life happens" would be quite the understatement. If I've neglected this thread, been short-tempered with any of you, or generally been a poor member of the group, I apologize. There's been a lot on my plate, but in the end that's no excuse. I promised that I'd take this project through to the end of the process described in Dan Raymer's textbook, and I will. It's just going to take a lot longer than I had expected. As I've alluded to in other threads, I've also started a shorter-term airplane design project intended to get an airplane designed and built "fastest for the leastest", and that's been taking away some of my available time as well. It's a higher priority under the circumstances. The poor DS54 motorglider has suffered neglect as a result, and I'm here today to give it a little love.

Working from my checklist in the last post, I've done the following:

* Decided on the NACA 0009 airfoil for the horizontal tail. This is a symmetrical airfoil that should give good service in this role. It's on the thin side for drag-reduction, but my intention is for this to be a full-depth foam-core flying surface, and so it's not going to be a real weight issue here.

* Decided on the NACA 0012 or even 0015 airfoil for the vertical tail. It's thicker than the horizontal tail for reasons that will immediately become apparent when I post the redrawn design. The final choice between the two will be structural.

* Decided that I am NOT going to include dedicated space for a parachute in the design. This was a tough choice. I want one, but like the Rutan Quickie from which this was conceptually derived, this is an airplane you "wear", not "get into". I don't do competition soaring, nor would this airplane be appropriate for competition soaring. It's for recreational use, and so there's just never any reason to "push it" in terms of terrain or weather to the point where a 'chute would be likely to be used. For initial flight testing, I'd probably leave out the seat cushioning and borrow one of the thin seat-pack 'chutes that extend from your shoulders down under your butt. Once flight testing and envelope proving are done, so the 'chute can go back to its owner with my thanks, and the airplane flown without one, just like any other non-aerobatic sportplane.

* Confirmed that the as-drawn fuel tank is big enough for the fuel load required. The required fuel for the design mission, including 6% unusable (per Raymer), is 9.8 US gallons. The as-drawn tank holds just under 9.5 US gallons. For a single tank like this, a 6% unusable margin is probably too conservative, and the crude "tank" included in the drawing doesn't have any kind of sump, the inclusion of which would easily add the "missing" 0.3 US gallons. Call it close enough for this stage of the work.

* Decided that the wing dihedral break happens at the fuselage centerline, not at the point where the outer wing panels remove from the stub-wings. This was mostly an aesthetic decision. There were some pragmatic reasons to make the dihedral break at the outboard location, but in the end I couldn't stomach the "look" in comparison to the rest of the airplane. Hey, looks matter too.

I have the airplane mostly redrawn from the previous sizing and the items above. There are two items from the list above that would still be "open", given the work above.

* I have not yet redrawn the firewall-forward section. I need to look up the dimensions of a real spinner I could really buy from Aircraft Spruce, and flesh out my engine drawing from the bare-bones diagram I got from Scott Casler at Hummel Engines to something more-representative of the real engine with all the intake, exhaust, and plumbing pieces in-place. I'm kind of waffling on this one, as the final cowling would literally be made around the real engine and a real spinner, and the amount of difference getting it nailed down to the nth degree on paper now would make, versus "winging it" in the shop, is likely very small. Dunno. I'll figure it out.

* The last item on my list was to try and get the fuselage bottom completely flat, to make the fuselage just that much easier to build. I've abandoned this idea completely. I reviewed the Rutan Quickie plans and build manual, drew a couple of flat-bottom variants of this design, and decided the slight improvement in "buildability" wasn't worth the butt-ugly result. Another case of aesthetics winning out.

Next Post: I'll make a final decision on the firewall-forward stuff, finish that work if I decide to do it, and then post the re-drawn design.

Super Moderator

All of the work I've done over the last few pages means that the drawing I've been posting is rather badly out of date. Many things have changed, become better-defined, etc. I'm about to launch into stability and control in a deeper fashion, so I need an updated drawing from which I can take dimensions for that work, reflecting all these changes. It'll be one of several times I redraw the airplane. Airplane design is an iterative process.

So, without further ado, here's the redrawn DS54, in my nomenclature, now DS54-2.

​

This is an exterior view with the landing gear retracted, and in full "soaring" mode with the plug-in outriggers removed. You can refer to the previous drawing to see the outriggers in place - they haven't changed since that drawing.

You'll note several large changes in the airplane since the last drawing. Let's go through them. First and foremost is the fuselage shape - swoopy! Part of this is aesthetic, and part functional. Aesthetically, this design was originally conceived as a Rutan Quickie with long wings and a conventional tail, and I happen to like the look of the Quickie fuselage a lot. The practical reason is that "tadpole" pressure-recovery fuselages like this are lower-drag than conventional "frustum" fuselages, and since this is a motorglider, every bit of drag reduction that can be pragmatically included counts. The final shape comes from a desire to create a pressure-recovery shape that's as easy-as-possible to build. I really have to admire how Rutan did it on the Quickie, and stole that part of his design baldly for my project. Essentially, this is a pressure-recovery fuselage with only one curved component, and that's the bottom. The turtledeck is flat-wrap, the sides are flat. Making the bottom curve, once you see it in the Quickie build manual, is simplicity itself. Clever work, Mr. Rutan!

The other big change is the vertical position of the horizontal tail. It's a T-tail now. Why? Because a T-tail, as done in quite a number of sailplanes, is super-easy to make removable. A couple of pins on the vertical tail slide into the elevator from its leading edge, and rotate on the same axis to actuate the elevator. Another pin, with a ball-detent, comes in from the leading edge of the stabilizer to fix the tail onto the vertical. Pull that one pin and the tail can be removed, yet it's all very secure and simple. (I'm making this post from a restaurant, and a web search only turned up R/C glider images, so I don't have a photo. I'll update this post if I can find one. Anyone who knows the mechanism I'm talking about and has an image, please PM me.)

I haven't redrawn the thickness of the vertical tail here - you'll recall that I just recently decided on a couple of airfoils for it, and won't make the final decision until much later, during structural analysis. So in this drawing, it's "too thin." That's okay for now.

Other changes. The canopy is quite a bit smaller, making it less expensive to have made, and also improving the grazing angle of the pilot's sightline. There's a risk in too small a grazing angle that the canopy becomes highly reflective and all the pilot can see is the glareshield. That's not nice, so a steeper front canopy takes care of that and also shortens the canopy, making it less expensive. I've changed the height of the canopy so that it's a semi-circle in cross-section, allowing it to be free-blown instead of molded. Another cost-saving measure.

The wings haven't changed (I confirmed my initial choice to have the dihedral break be on the airplane centerline), and my first, crude, tail-volume calculations proved remarkably accurate - the tails are almost unchanged in size as well. That may change with more-comprehensive stability and control work. We'll see.

The rest of the changes are internal. Here's the new inboard profile of the cockpit area:

​

I've included pilot "dummies" for the full range of design pilot heights: 5'4" to 6'3". The baseline pilot (me) is 5'10". Crashworthiness issues and design decisions are discussed exte:speechlesnsively above, and those decisions are reflected in the drawing. Seatbelts and harnesses are drawn in, to show their attachments points for later structural work. Please see that earlier discussion for details. One item marked on the drawing is the 30" "flail" distance from the back of the smallest pilot's head forward, showing clearance from the instrument panel. This is a recommendation from my crashworthiness reference, also discussed above.

Also indicated here are the fuel tank (see the immediately previous post), the centerline main gear struts and retracted position (between the pilots legs, like a Fourier RF-4), and the baggage space behind the pilot. The latter shows volume for a US standard "carryon" bag, even though what will actually be placed there is almost certainly a "day pack" type bag. This airplane isn't intended for anything more extensive than overnight or weekend "bachelor" trips to soaring events and airshows.

So there it is. Still early on, but the design is shaping up nicely. I hope you like it. I do!

Next Post: Begin analysis of the redrawn design for drag, stability, and control.

Super Moderator

Almost two years. >sigh< It's been almost two years since I've posted here on this project. Much has happened, and unfortunately a much smaller fraction of that than I'd like has been "moving the project forward." The last five years have been among the most challenging of my life, and those challenges have kept me from participating here much, and from doing a lot of meaningful work on this project.

So, in brief, here's what's been going on with this little motorglider project.

At the time of my last post, I was in the middle of the homebuilding version of a "design review", taking stock of where the design was at the time and looking at how it matched the initial goals and specifications.

Two words: "Scope creep."

The point of origin of the project was a visit to Harbor Freight with a friend of mine who is equally interested in homebuilding, and standing at the row of small industrial motors they sell. We were both of the opinion that it would be really neat to build a tiny quick-build airplane, in the vein of the Rutan Quickie concept. From there, you've seen the progress of the airplane to something powered by a 1/2 VW engine over five times as expensive, with soaring performance exceeding 30:1, flaps, drag brakes, and so on. The process of scope creep is insidious. It's so easy to "just make it a little better" here and there, until what you're designing is a lot more sophisticated and expensive than the original goals. I realized that that's what had happened here. I'd gone from "tiny, quick-build Quickie with long wings" to "relatively high-performance touring motorglider." I didn't rip everything and start over, but fixing things was a very major "iteration" on the project.

First off, I reaffirmed my basic requirements set and specifications. I decided that, yes, I'd done a pretty decent job with those, but that build simplicity and low-cost needed to be higher-priority goals than hitting the top end of all my performance goal ranges. Trying to achieve all the latter is what made the airplane heavier, more complex, and more expensive.

I also stepped back and revised my design process. Raymer's full method is awesome for airplanes that fit in categories that have quite a few examples, from which a meaningful database of design parameters can be drawn. Very small single-seat touring motorgliders aren't well represented, and most of the designs (as you've seen from my listing early in the design study) are pretty old and widely varying in construction methods and materials. So, back to first-principles. Don Crawford's book, A Practical Guide to Airplane Performance and Design is a more-basic design process based on the method shown in Perkins & Hage's classic text. It doesn't do a design in as much detail as Raymer's process, but it also is more of a pure numeric process, without the need for a database of comparable designs. I've been using Crawford's equations and then using his nomograph sheets as a quick check on my math. Overall, I like that his method allows for pretty rapid iteration of a design and gets the basic numbers not only into the ballpark, but into the infield, as it were. Raymer would get you onto the pitcher's mound, but for this project that seems like overkill, given the dearth of data it needs for input.

All of this is background for what's come out of the process, which I'll describe now.

The airplane isn't really that different. Overall configuration is the same. The "skeg" at the bottom of the vertical stabilizer is gone. That's the biggest visual change. Main structure is still Rutan-style moldless composites, with the structure of the fuselage heavily based, in concept, on the Rutan Quickie. I don't know if the main gear will be retractable or not. I'm still tossing that over in my mind. If fixed, it will be faired and the single main wheel enclosed in a pant.

The engine is a Harbor Freight Predator 670cc V-twin. That's the biggest major change. The engine puts out 22hp in stock trim. I've lurked, and occasionally commented, in the discussions on industrial V-twins here on HBA over the last couple of years. My philosophy is to do as minimal a "conversion" as possible, and design the airplane to fly adequately on 22hp rather than try and get more power out of the engine. While I don't yet have all the details worked out, my idea of converting the Predator 670cc engine for this project is to remove the shrouds and stock muffler, re-locate the oil cooler into a position with better airflow, remove the flywheel and fit a new set of ignition-trigger magnets (I've provided a link to an available kit for that, priced at about $100, designed for this engine). The prop mounts, direct-drive, to the flywheel end, on a short (4-5") extension. Fitting a CV carb seems like a good idea, but I haven't looked into it. And that's the extent of "conversion" for me. I think doing much more obviates the advantages of using an industrial motor in the first place. YMMV, of course.

Rather than a curve-fit comparison to similar designs as in Raymer, I've used some more direct methods to estimate MTOW at 550# and We at 283#. The lower weight means a smaller wing, as follows: 63 square feet, 35' span, AR=19.4. The shape is the same double-taper with a straight trailing edge. Basically a smaller version of the wing you saw earlier.

An interesting problem arose when the FX79 airfoil that I'd chosen earlier proved to be a poor choice for this smaller wing, due to Reynold's Number considerations. The operating Re at the wingtip, at stall at 5,000'MSL, is as low as about 395,000. Wow. So I cast about for a new airfoil that meets my revised design requirements, and ultimately landed on a modified version of the Eppler 361 airfoil. This was originally designed as an advanced helicopter blade airfoil, which means it has very low pitching moment, decent maximum lift, quite good drag, and works well at low Reynold's Numbers. My modification was a small amount of additional camber, to increase the Clmax. I verified the modification in Xfoil, both that the lift increased as desired and that the other characteristics are largely unchanged.

One of the reasons that I'm still waffling on retracting the main landing gear is that the airplane is intended to fully comply with the LSA specifications. LSA "Airplanes" must have fixed landing gear, but LSA "Gliders" can have retracts. Unfortunately, the LSA rules don't quantify "powered gliders" even as well as Part 23 does (which isn't very well itself), and right now there doesn't seem to be any consensus whether a powered glider LSA can have retractable main gear or not. Assuming the FAA treats the relationship between LSA powered gliders to LSA airplanes and unpowered LSA gliders the same as it does to their Part 23 equivalents, then an LSA powered glider should be able to have retractable gear. The speed limits for powered flight would still apply, but soaring performance would improve.

Speaking of performance, my numbers are coming out as follows. A stall speed of 50 mph. According to the performance curves I have, top speed at sea level is about 143 mph, but that's for an "ideal" and variable-pitch prop. In the real world with a fixed-pitch wooden prop, I expect top speed to be in the vicinity of 110 mph. Cruise would be around 100 mph. Best L/D is about 28:1 and minimum sink is about 170 fpm, both assuming a feathering prop.

All in all, this new iteration better meets my original goals, of a very small, quick- and inexpensively-built single seat motorglider, suitable for short (200-250 mile) powered cross-country, moderate-performance thermal and good-performance ridge soaring, and general time-building. I don't have a real drawing of the new airplane, but as I mentioned earlier, knock the lower "skeg" off the vertical stabilizer on the last drawing, and you'll very much be in the ballpark.

Apparently it is a still unresolved glitch in the new software. Following your lead I set up a build and comment thread. The build thread has a bunch of comments in it now that kind of messes up the continuity. I was even locked out of posting on my own log for a while. Admin managed to fix that.......

I empathize about the "life" problems. Keep the goal in mind!

Edit: Just realized I did the same to your build log. Please move - if you can.